So-called "hard" contact lenses and compositions for fabricating them are well known in the art. The standard contact lens used for many years was fabricated from polymethyl methacrylate (PMMA) and achieved widespread use throughout the world, despite the fact that a PMMA lens is essentially impermeable to oxygen. The lack of oxygen at the interface between the contact lens and the cornea will, after even short periods of time, i.e., less than one day, cause discomfort to the wearer because of edema induced by the lack of oxygen at the surface of the cornea. Consequently, PMMA lenses must be removed at least daily and preferrably more often to expose the surface of the cornea to ambient atmospheric oxygen and thereafter replaced, a bothersome procedure.
In an attempt to remedy these defects inherent in standard PMMA contact lenses, the art devised other copolymer contact lens compositions having improved oxygen-permeability which allow the user to wear the lenses for longer periods of time, sometimes several days, before they must be removed for cleaning. These oxygen-permeable lenses are of two general types, the so-called "soft" lens formed of the very flexible Hydrogel.RTM. material and the so-called "hard permeable" lens formed of a solid copolymer of methyl methacrylate with various comonomers. The hard permeable lens avoids certain problems associated with the so-called soft lens, principally in its resistance to contamination and breakage, and provides ease of cleaning and resistance to surface scratching.
Although the hard permeable contact lens compositions represented a marked improvement over standard PMMA polymers in terms of oxygen-permeability, they nevertheless suffered certain disadvantages in comparison to standard PMMA lenses in terms of wettability, mechanical stability and chemical stability. Moreover, certain of the present day hard permeable copolymers require special surface pre-treatment to improve wettability and/or special lens treatment solutions which are used throughout the life of the lens to improve or maintain surface properties. Furthermore, the hard permeable copolymers are inherently brittle with the result that they frequently chip at their edges and break. Such chipping obviously creates a potential health hazard to the wearer and imposes a definite economic burden because they must be replaced.
This invention will be best understood by reference to the prior art described in Keogh, U.S. Pat. No. 4,259,467, and Ratkowski et al, U.S. Pat. No. 4,419,505, and the several references cited therein. Further background is available from Gaylord, U.S. Pat. No. 3,808,178; Ellis et al, U.S. Pat. No. 4,168,122; Ellis et al, U.S. Pat. No. 4,152,508; Novicky, U.S. Pat. No 4,216,303; Novicky, U.S. Pat. No. 4,242,483; Neefe, U.S. Pat. No. 4,306,042 and Foley, E.P. No. 75004.